KFL@MC.LCS.MIT.EDU ("Keith F. Lynch") (02/12/86)
From: shawn@acc.arpa
While most of the talk of SDI is mostly over my head, I've also been
reading the messages about "laser powered" space flight. In space
flight, a laser is used to push a space ship from earth.
Not exactly. The laser is used to vaporize reaction mass in the
rocket. Unless you are talking about a REALLY REALLY POWERFUL LASER
and a lightsail, as has been suggested by Robert Forward.
In SDI a laser is bounced off of an object tthat is amed elsewhere. OK, so
will the laser push the mirror out of its position with each recoil
and if so, would that push be directed by the angle of the reflected
shot?
The momentum of light is very small. It is equal to the energy
content of the light divided by the speed of light. 300 megawatts of
light would only impart one kilogram meter per second per second of
momentum to the laser. If the laser satellite has a mass of 1000
metric tons, and the laser emits 100 megawatts of light for 100
milliseconds, the velocity of the satellite will change by only
3*10^-8 meters per second, i.e. ONE METER PER YEAR.
I am an SDI researcher with a defense firm. (Usual disclaimers
apply.)
...Keith